In a coating operation, a solvent-type coating agent or paint, consuming a larger quantity of an organic solvent, allows a larger amount of the solvent to be dissipated to atmosphere to give rise to atmospheric pollution or worsening of the working environment. Thus the use of the aqueous paint is considered and, above all, the application of the aqueous paint to electrostatic coating is implemented.
With continued use of the electrostatic coating device, a solidified mass, either dried or semi-dried, of the coating solution is formed on an electrostatic spray head, such as a nozzle, bell, spray gun or disk, atomizing the coating solution, above all of an aqueous type, and grows in size. The solid mass that has grown in size obstructs atomization of the coating solution to produce unevenness in the coating film or becomes affixed to the article to deteriorate the appearance and quality of the coating film. Thus the conventional practice has been to supply cold or warm water or other liquids to the coating head directly to dissolve the solid mass before it grows in size to deteriorate the appearance of the coating film. In the JP U.M. Kokoku Publication No. 62-31174 (1987), there is proposed a spray coating device in which cold or warm water is caused to flow on a liquid film forming plate disposed in the vicinity of the article to be coated to prevent the growth of the solid mass.
However, when cold or warm water for dissolving the solid mass is supplied directly to the electrostatic coating head, it is necessary to cut off the high voltage source for applying a high voltage to the coating head to prevent short-circuiting. Moreover, a large quantity of water is required for cleaning with cold or warm water resulting in that much time is involved in cleaning the coating head.
In addition, safety problems may be presented when the operator forgets cutting off the high voltage source when cleaning the spray head.
The problems inherent in the electrostatic spray device provided with a cup-shaped rotary atomizing head as the electrostatic spray head and the measures taken in the prior art to cope with these problems, are discussed hereinbelow.
In the electrostatic coating device, having the cup-shaped (or so-called mini-bell type) rotary atomizing head, the paint supplied from a paint supply nozzle to a paint receiving portion is in the form of a thin film, under the centrifugal force produced by rotation of the cup-shaped rotary atomizing head, and is discharged and atomized by the rotary discharge rim. However, the following problem may be presented when the electrostatic coating method is used to atomize the aqueous paint which is rich in volatile solvents and in which the boiling point is difficult to control.
When the rotary atomizing head is rotated at a speed of about 4000 r.p.m. or higher, a reduced pressure is created in the inner space of the atomizing head to produce an air pumping phenomenon, that is, a phenomenon in which air is driven into the inside of the atomizing head. A part of liquid droplets of the comminuted aqueous paint discharged from the rotary discharge rim is entrained in the air driven into the inner space of the atomizing head to become affixed to the inner peripheral surface of the atomizing head. At the partial place where the this film of the aqueous paint has not been formed on the inner peripheral surface, the moisture in the deposited liquid droplets is vaporized and the liquid droplets are dried or semi-dried to become affixed to the atomizer head in the form of precipitates of the solid component. When the aqueous paint is continuously atomized for a prolonged period of time in this state, the solid components continue to be precipitated and accumulated to grow in size in the normal line direction until they come to obstruct the flow of the water base paint in the from of a thin film. If atomization is continued further, liquid droplets become affixed to the discharge rim of the inner peripheral surface where the flow is obstructed to form precipitates of solid components which then are allowed to grow towards and as far as the discharge rim. Thus the aqueous paint cannot be formed into a uniform thin film while it is guided as the thin film towards the discharge rim. Thus, when the atomizing head is used in a coating device, the atomized aqueous paint has a larger particle size to lower the coating performance. When the head is used in a coating device employing a pneumatic motor, the number of revolutions of the atomizing head is lowered on account of the precipitates solid components to give rise to similar inconveniences.
In addition, a part of the liquid droplets of the comminuted aqueous paint discharged from the rotary discharge rim is affixed in a larger amount on the reverse side of the discharge rim of the atomizing head to cause solid components to become precipitated to grow in size in the normal line direction or occasionally to become affixed on the article to be painted to deteriorate the coating quality or performance.
For overcoming the above mentioned drawbacks, there are disclosed in the JP Patent Kokai Publications Nos. 57-24660, 57-24661, 57-24672, 57-24673 and 57-24674 (all of 1982) an electrostatic coating apparatus in which, when electrostatically atomizing the water base paint at the rotary discharge rim for electrostatic coating, water, lukewarm water, water-soluble solvents, solvents or a mixture of water and a solvent, is supplied to the cup-shaped rotary atomizing head to effect cleaning of the rotary cup.
In the JP Patent Kokai Publication No. 54-154436 (1979), there is disclosed a cleaning method and a cleaning device in which automatic grounding is effected by a grounding device at the same time that the voltage applied to a rotary cup is interrupted during cleaning of the rotary cup, wherein the charges on the cup are allowed to discharge without sparking.
Various methods and devices for cleaning the cup-shaped rotary atomizing head are also known such as by the JP UM Kokai Publications Nos. 61-106360, 61-106361 and 61-106362 (all of 1986).
However, these methods are inconvenient in that the high electrical voltage applied to the rotary cup has to be interrupted once during the coating operation to protract the total cleaning time, and in that, in case of continuous coating or coating on a continuously running article, the output per unit time may be lowered due to increase in insufficiently cleaned articles.